Abstract

We propose two designs of effectively single mode porous polymer fibers for low-loss guiding of terahertz radiation. First, we present a fiber of several wavelengths in diameter containing an array of sub-wavelength holes separated by sub-wavelength material veins. Second, we detail a large diameter hollow core photonic bandgap Bragg fiber made of solid film layers suspended in air by a network of circular bridges. Numerical simulations of radiation, absorption and bending losses are presented; strategies for the experimental realization of both fibers are suggested. Emphasis is put on the optimization of the fiber geometries to increase the fraction of power guided in the air inside of the fiber, thereby alleviating the effects of material absorption and interaction with the environment. Total fiber loss of less than 10 dB/m, bending radii as tight as 3 cm, and fiber bandwidth of ~1 THz is predicted for the porous fibers with sub-wavelength holes. Performance of this fiber type is also compared to that of the equivalent sub-wavelength rod-in-the-air fiber with a conclusion that suggested porous fibers outperform considerably the rod-in-the-air fiber designs. For the porous Bragg fibers total loss of less than 5 dB/m, bending radii as tight as 12 cm, and fiber bandwidth of ~0.1 THz are predicted. Coupling to the surface states of a multilayer reflector facilitated by the material bridges is determined as primary mechanism responsible for the reduction of the bandwidth of a porous Bragg fiber. In all the simulations, polymer fiber material is assumed to be Teflon with bulk absorption loss of 130 dB/m.

Figures (7)

Schematics of two porous fibers studied in this paper. a) Cross-section of a porous fiber with multiple sub-wavelength holes of diameter d≪λ separated by pitch Λ. b) Cross-section of a porous Bragg fiber featuring periodic sequence of concentric material rings of thickness h suspended in air by a network of circular bridges of diameter drod.

a) Effective refractive index of the fundamental core mode versus d/Λ for the two fiber designs having hole diameters of d/λ=[0.1,0.15]. For the fiber with d/λ=0.1, distribution of the power flux in the waveguide crossection Sz is shown for d/Λ=0.8 in the inset (a). b) Fraction of modal power guided in the air as a function of d/Λ. The two upper curves show the total power fraction in the air (air plus cladding) while the two lower curves indicate the power fraction in the air holes only.

Comparison of the propagation characteristics of the fundamental mode of a porous fiber (solid curves) with those of the fundamental mode of the equivalent rod-in-the-air subwavelength fiber (dashed curves). a) Normalized fiber and mode diameters. b) Modal effective refractive indices. c) Modal losses due to macro-bending.

Various implementations of porous fibers. a) Increasing the number of layers in a porous fiber leads to modes with larger effective mode diameters. In the lower plot a typical performance of a 4 layer porous fiber designed for λ=300 µm is shown. b) Schematic of a 25 layer porous Bragg fiber and flux distribution in its fundamental mode.

Radiation losses (solid lines) and absorption losses (dashed lines) of the hollow core Bragg fibers for various bridge sizes drod=[100,200,300] µm. For comparison, radiation loss of the equivalent Bragg fibers without rods are presented as dotted lines. Inset II shows Sz flux distribution in the fundamental core guided mode positioned at the minimum of the local bandgap at λ=378 µm. Insets I and III show field distributions in the fundamental core mode at the wavelengths of coupling with different surface states.

Bending losses of a porous Bragg fiber without bridges designed and operated at λc=300 µm. Bending loss is strongly sensitive to the polarization of an HE11 mode, with the polarization in the plane of a bend being the lossiest. In the insets we show Sz flux distributions at the output of the 90° bends of various radii.